The latest report by Tokyo Electric Power Company (TEPCO) has shed more light on the 2011 Fukushima nuclear disaster, revealing a failure in one of the reactors' pressure relief systems.

The new report looks into data discovered by a
robot that ventured into the Fukushima Daiichi Reactor-2 building
in October. It measured radiation levels at various places and
studied the conditions of leakage control systems.

The main focus of the investigation is the reactor's containment
system, which is its last line of defense against radioactive
material leakage. It is a shell built around the reactor and
equipped with a system of pipes and valves to release excess
pressure. It is a last-resort measure that vents radiation into
the atmosphere to prevent more serious accidents.

Following the March 2011 magnitude 9.0 earthquake, pressure was
rising at the crippled reactor, and staff opened the valves to
send excess contaminated steam through a pipe to an exhaust
tower.

However, the system failed. It features an element called a
"rupture disk" between the valves and the exhaust pipe, which is
supposed to burst under enough pressure and allow the steam
through. But robot readings showed no increase in radiation
levels near the disk, which means it never did burst.

The TEPCO report suggests that gases from Reactor-1 flowed
back into Reactor-2's system, pressuring the disk from the
opposite side. Both reactors use the same exhaust pipe in their
venting systems.

The report's diagrams also show that only about 25 percent of the
valves were found open. TEPCO, however, states that the findings
are "not definitive" and more inquiries will be needed to find
out exactly what happened.

TEPCO, the Fukushima plant operator, has used 16 robots to
explore the crippled plant to date, from military models to
radiation-resistant multi-segmented snake-like devices that can
fit through a small pipe. However, even the toughest models are
having trouble weathering the deadly radiation levels: the latest
robot sent into Reactor-1 broke down three hours into its planned
10-hour foray.

TEPCO is now on a lengthy mission to clean up the consequences of
the March 2011 disaster, when an earthquake damaged the plant,
causing several explosions and meltdowns and contaminating the
surrounding area.

The company also hopes to use the experience to improve safety at
its Kashiwazaki-Kariwa nuclear plant to restart operations there.
Kashiwazaki-Kariwa was shut down following the Fukushima
disaster.